Completed two system furnace maintenance and inspection. I tested amp draws and components. I cleaned flame sensors and wiped down furnaces. I tested temp splits and airflow. Basement furnace was overheating due to a clogged filter. Customer had filters there in the basement so I replaced the filter. I had to adjust gas pressure on the basement furnace as well. Attic furnace did not have a filter in the filter slot. I found a filter that fit into the slot. I advised that customer gets a Merv six filter for the unit to provide the most amount of airflow. The attic furnace was also overheating. I had to adjust gas pressure on that system as well. Both furnaces are operating properly at departure.